This invention relates to green-emitting phosphors suitable for high-density electron bombardment and a cathode-ray tube using these phosphors. The density of electric current used in projection cathode-ray tubes is more than 100 times as large as that of electric current used in direct-view tubes. For this reason, phosphors used in the projection tubes are not only required to have high emission efficiency but also required to have excellent current and temperature characteristics of brightness and cause little change of brightness with irradiation time even in high-density electron bombardment.
In particular, it is desired that green-emitting phosphors, which contribute to white brightness of a screen the best, have excellent emission characteristics. Under these circumstances, many manufacturers are trying to improve phosphors and studying various materials, particularly, for green color, as is described in Proceedings of the 217th Meeting of Phosphor Research Society; Phosphors for projection tubes, pp. 19.about.26 (1987) .
For example, Japanese Patent Kokai (Laid-Open) No. 37670/1973, U.S. Pat. No. 3,758,413, etc., disclose a phosphor having a composition of Y.sub.2 SiO.sub.5 : Tb.
Japanese Patent Kokai (Laid-Open) No. 101175/1985 discloses a phosphor having a composition of Y.sub.3 (Al.sub.1-x Ga.sub.x).sub.5 O.sub.12 : Tb (wherein 0.ltoreq.x.ltoreq.1). This phosphor has current characteristics of brightness that brightness increases generally in proportion to a current density at a low current density (of not more than 10 .mu.A/cm.sup.2) and excellent temperature characteristics of brightness that brightness changes little against a temperature change of the phosphor. Thus, this phosphor has high emission efficiency. However, the problem of this phosphor is that at a high current density (of more than 10 .mu.A/cm.sup.2), brightness does not increase in proportion to a current density but nearly shows a saturation tendency. The "current density" above stands for a value obtained by dividing an electron beam current by a raster area. Further, U.S. Pat. No. 3,666,676 discloses a phosphor represented by general formula M.sub.1-x-y OX: Tb.sub.x Yb.sub.y wherein M denotes an element selected from a group consisting of La and Gd, and X denotes an element selected from a group consisting of Cl and Br. The above U.S. Patent describes that afterglow in a unit of second, observed after X-ray excitation, is decreased by adding Yb. However, all of the above conventional green-emitting phosphors are colored under high-density electron bombardment and cause problems of deterioration of emission efficiency and brightness with irradiation time (so-called aging). For example, according to an experiment of the present inventors, when the ratio of brightness after electron bombardment to initial brightness (referred to as brightness maintenance factor) in relatively stable Y.sub.3 Al.sub.5 O.sub.12 : Tb is 0.95, that in LaOCl: Tb, Yb is 0.5 to 0.6. In order to make green-emitting phosphors usable practically in a projection cathode-ray tube, therefore, first of all, brightness deterioration due to electron beam irradiation needs to be prevented, and further, it is necessary to prevent brightness saturation and maintain high emission efficiency even under high current density.
As for codoping of Tb with Eu, and Japanese Patent Kokai (Laid-Open) No. 105075/1984 and U.S. Pat. No. 3,394,048 disclose an orange-emitting phosphor having a composition of InBO.sub.3 in which Eu and Tb are co-activated.